Sheet metal forming dies



Jan. 21, 1969 K. F. JAMES ET AL 3,422,663

- SHEET METAL FORMING DIES Original Filed Aug. 22, 1963 GENERATGQ 77'\ WATER rmp ll CONDENSER CUMPRESSOR HEAT {Q (F10 6 1? EXCHANGER W i fiL ll r 11 l 12 I INVENTORS 2 %wzei'/ c/zzzes A T ORNEY United States Patent 3,422,663 SHEET METAL FORMING DIES Kenneth F. James and Alexander H. Joyce, Detroit, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Original application Aug. 22, 1963, Ser. No. 303,783, now Patent No. 3,313,007, dated Apr. 11, 1967. Divided and this application Dec. 22, 1966, Ser. No. 603,937 US. Cl. 72-476 4 Claims Int. Cl. B21d 37/00 ABSTRACT OF THE DISCLOSURE A cast-to-size sheet metal forming die is disclosed which is made of steel particles brazed together with a copper base alloy by preparing a mold from nepheline syenite sand and a binder which mold has a coeificient of thermal expansion substantially equal to the coefficient of thermal expansion of the steel particles over a temperature range from about room temperature to a temperature above the pouring temperature of the braze alloy, filling the mold with steel particles and shot of varying sizes, and casting with brazing alloy. The steel particles in the body portion of the die are a mixture of relatively large and small particles and the particles in the workface portion are all relatively small.

This application is a divisional application of our copending application S.N. 303,783 which was filed Aug. 22, 1963, now Patent No. 3,313,007.

This invention relates generally to casting metal articles. More particularly, this invention relates to precision cast-to-size brazed shot dies and to a method of making same.

Most commonly used casting techniques for casting metal articles, such as sheet metal forming dies, involve the step of pouring molten metal, such as cast iron or steel, into a mold cavity which is contoured to the general size and shape of the article to be produced. Molds made of sand or similar ceramic materials are widely used in metal casting processes, since ceramic molds are relatively inexpensive and easy to fabricate. Most commonly used ceramic molds have fairly good structural strength at the relatively high temperatures employed in most commonly used iron or steel casting processes, and also they are relatively resistant to the diffusion of the molten metal into the walls of the mold cavity during the casting process. However, casting processes, in which metal articles such as cast iron or steel dies are cast in ceramic molds, usually involve problems of size and shape control in forming the article, since the articles which are formed by these processes generally have a somewhat ditlerent size and shape than the initial size and shape of the mold cavity.

The problem of controlling the final size and shape of the metal article, such as a steel or cast iron die, formed by most commonly used casting processes generally is attributed to the difference in the characteristics of thermal expansion of the ceramic mold and the characteristics of thermal and solidification shrinkage of the metallic die. In this application, by thermal expansion characteristics of the mold, we refer to the manner in which the mold cavity becomes enlarged and deformed from its initial size and shape on being heated during the casting process. By thermal shrinkage characteristics of the metallic die, we refer to the manner in which the molten metal materials in the mold cavity solidify and contract on cooling to form the die. In other words, shrinkage due to solidification as well as thermal contraction is included in the term thermal shrinkage characteristics. Thus, since the thermal expansion characteristics of a ceramic mold usually differ from the thermal shrinkage characteristics of a metallic casting, metal dies which are made by a casting technique utilizing ceramic molds generally have a different size and shape than the initial size and shape of the mold cavity. Also, when irregular shaped metal articles or dies are cast in a mold, differential solidification rates between the heavy and light sections of the castings usually cause a considerable amount of distortion. In fact, this latter problem is often the major cause of distortion in casting a metal article or die.

In the past these problems have been compensated for by providing a mold cavity which is somewhat larger than the desired final size of the die to be produced and by casting an oversize die. The excess stock on the die so formed is subsequently removed from the cast die by post-machining operations, such as kellering and barbering, to conform the die to the desired final shape and size. However, these post-machining and handling operations are relatively expensive and difiicult to perform. This is particularly true in casting metal dies for use in sheet metal forming operations, since the size and shape of these dies must be very accurately controlled if the dies are to operate properly in a sheet metal forming press or similar apparatus. Naturally, the post-machining operations and handling steps which are involved contribute greatly to the overall cost of the die making process.

Thus it is desirable to form precision cast-to-size metal dies so that the number of post-machining operations required is substantially reduced, thereby reducing the costs of the die making process. In this application, by precision cast-to-size dies, yve refer to dies which are formed by a casting process in which the cast die has substantially the same size and shape as the initial size and shape of the mold cavity. In addition to the advantages that this type of precision casting process offers by eliminating or greatly reducing the number of necessary post-machining operations, it also otters other process advantages. For instance, such a process eliminates the necessity of forming an enlarged mold cavity and permits the direct use of the die pattern or master model, which are exact duplicates of the die to be produced, to form the mold cavity. Hence, the die which is produced in the mold cavity will have substantially the same size and shape as the master model or die pattern used to form the mold cavity.

The present invention involves a precision casting process utilizing a brazing procedure to form metal ar ticles, such as brazed shot dies. We have found that the brazed shot dies so produced generally have excellent Wear resistance which in many cases is superior to commonly used dies Which are cast from steel or cast iron. In addition, as will hereinafter be more fully explained,

we have found that the process of the present invention, by which small metal shot particles are brazed together to form brazed shot dies, substantially eliminates the problems of solidification shrinkage previously mentioned in the case of casting irregular shaped metal articles having heavy and light sections.

However, as is well-known in the art, most brazing techniques used in making metal articles similar to the brazed shot dies of the present invention generally involve the used of more expensive process steps, equipment and materials than the processes, equipment and materials used in most commonly used casting processes to cast metal articles, such as cast steel or cast iron dies. For instance, brazing techniques often require the use of expensive furnace equipment having a special protective furnace atmosphere to prevent oxidation of the brazing alloy or metal parts which are being brazed together. Also, the brazing alloys and metal shot and filler particles used to form the brazed shot dies of the present invention are generally more expensive materials for use in forming a die than regular tool steel or cast iron.

In addition, even if the previously mentioned problems of differential solidification is substantially elminated by casting a metal article in a ceramic mold using a brazing alloy to bond metal particles together in the mold cavity, the thermal expansion characteristics of the ceramic mold must be compensated for in order to achieve a precision cast-to-size process for making dies. Otherwise, the number of post-machining operations necessary to conform the die to the desired final shape and the resulting fabrication costs would not be substantially reduced. Thus, if the number of necessary post-machining operations is substantially reduced by providing a process to make precision cast-to-size brazed shot dies, it is possible to greatly reduce the overall cost of fabricating brazed shot dies. Accordingly, the reduction in post-machining costs realized by using a precision casting process to form brazed shot dies would make the use of these dies economically competitive in most applications with the use of cast steel or cast iron dies which are made by most commonly used casting processes and which require substantial post-machining operations. In addition, the superior wear resistance and long life of brazed shot dies offer additional operational advantages over the use of dies cast from steel or cast iron in many applications, such as sheet metal forming processes. A process for making precision cast-to-size brazed shot dies has not heretofore been recognized.

Therefore, it is a principal object of the present invention to provide brazed shot die compositions and ceramic mold compositions having compensatory thermal expansion and shrinkage characteristics for use in a casting process to produce precision cast-to-size brazed shot dies.

It is another object of the present invention to provide a method of producing precision cast-to-size brazed shot dies to greatly reduce the number of post-machining operations which are required to conform the dies to the desired shape and size, thereby reducing the overall costs involved in making sheet metal forming dies.

It is a further object of the present invention to provide a method of producing precision cast-to-ize brazed shot dies having high strength and wear resistance for use in a sheet metal forming press assembly.

In accordance with the present invention, these and other objects are accomplished by providing a mold cavity in a ceramic mold having thermal expansion characteristics, over the temperature range employed in the casting process, which substantially compensate for the thermal shrinkage characteristics of the brazed shot die composition which is cast in the mold cavity to make the die. The process of making the precision cast-to-size brazed shot dies of the present invention involves the steps of providing a master model having the exact size and shape of the die which is to be produced, positioning the model in a suitable mold assembly, pouring a suitably mixed mold composition into the mold assembly over the model, curing the mold composition to form a ceramic mold and subsequently removing the cured mold from the mold assembly and the model from the mold to provide the mold cavity.

Metal shot and metal filler particles are next placed in the mold cavity in the desired arrangement, and the mold is placed in a suitable furnace and heated in a suitable protective atmosphere to an elevated temperature level to expand the mold cavity above a certain minimum size. When the cavity becomes sufficiently enlarged, a molten brazing alloy is poured into the mold cavity over the heated shot and filler particles to fill the voids therebetween, and the ceramic mold and metal composition in the mold cavity are subsequently cooled in the furnace so that the alloy solidifies and brazes the metal shot and metal particles together to form the brazed shot die embodying the present invention. On cooling, the die compositions of the present invention solidify and contract, thereby shrinking to the same extent as the mold cavity expands on heating, so that the brazed shot die which is formed has substantially the same size and shape as the initial shape of the mold cavity when the model is removed. The precision cast-to-size brazed shot die is subsequently removed from the mold cavity. Since the die which is formed by the process of the present invention has substantially the same size and shape as the model used to form the mold cavity, the number and extent of post-machining and handling operations necessary to conform the die to the desired final shape is greatly reduced.

Other features and advantages of the present invention will be apparent from the following description of certain embodiments thereof, taken in conjunction with the accompanying drawing, in which:

FIGURE 1 is a perspective view, with parts broken away, of a mold assembly which may be used to form the mold embodying the present invention;

FIGURE 2 is a vertical elevational view, with parts broken away and in section, of a furnace assembly and appurtenances which may be used in accordance with the process of the present invention to make the precision cast-to-size brazed shot dies of the present invention; and

FIGURE 3 is a vertical elevational view, with parts broken away and in section, of a precision cast-to-size brazed shot die embodying the present invention.

Referring to FIGURE 1 of the drawing, a typical mold assembly is illustrated which may be used to form the ceramic mold embodying the present invention. Four rectangular, plate-like wall members 10 are vertically mounted on a flat, rectangular base member 12 to form a cubical mold cavity 14 which opens upwards. The wall members and the base member may be made of wood or any other suitable material. These members may be detachably but firmly secured to one another by any suitable means, such as dowel pins, which are not shown in the drawing.

A master die pattern or model 16 having a contoured upper surface 18 which exactly duplicates the size and shape of the die to be produced, and a flat base surface 20, is suitably positioned in the cavity 14 so that the fiat base surface 20 of the model is firmly positioned on the flat base member 12, and the upper contoured surface of the model protrudes into the cavity. As previously mentioned, the precision casting process of the present invention permits the direct use of a duplicate model of the die to be produced, since the necessity of casting an oversize die is eliminated. Naturally, this is an important advantage of the present invention. The model may be made of plaster or any other suitable material, such as plastic.

Prior to forming the mold, the upper contoured surface 18 of the model preferably is treated in a conventional manner with any suitable parting agent or mold release agent, such as a silicone oil, to facilitate removal of the model after the mold has been formed. We have found that a parting agent which is commercially marketed under the trade name Aristo No. 5950 is suitable for use with a plaster model and the ceramic mold compositions of the present invention.

As previously mentioned, the present invention contemplates the use of mold compositions having thermal expansion characteristics which substantially compensate for the thermal shrinkage characteristics of the brazed shot die compositions of the present invention, so that the aforementioned problems of size and shape control, which arise due to differences in these characteristics, are greatly reduced or eliminated. In the present invention, we contemplate the use of a mold comprising, on a weight basis about 80% to 95% nepheline syenite sand as the base material with about 3% to sodium silicate and 2% to 10% calcium aluminate as binders. In the preferred embodiment of the present invention, the mold has a composition, by weight, of about 84% to 91% nepheline syenite, about 5% to 8% calcium aluminate and about 4% to 8% sodium silicate.

Nepheline syenite is a commercially available ceramic material which consists essentially of a mixture of several different sodium aluminum silicates and potassium aluminum silicates. A typical analysis of nepheline syenite on a volumetric percent basis which is suitable for use in the process of the present invention is listed in Table I below:

TABLE I Percent Mineral composition of nepheline syenite: by vol. Al'bite 54 Microcline 20 Nepheline 20 Mucovite 2 Mafics 2 Balance (biotite, Hastingsite, magnetite) 2 Total 100 In the preferred embodiment of the present invention, we contemplate the use of nepheline syenite having a mesh size ranging from 20 to 40 mesh. Naturally, it will be appreciated that the mesh size and the mineral cOmposition of the nepheline syenite may be varied somewhat from the composition listed in Table I and the mesh size specified in the preferred embodiment of the invention. Of course, since these variations or changes will have some effect on the thermal expansion characteristics of the molds used in accordance with the process of the present invention, corresponding changes must also be made in the thermal shrinkage characteristics of the brazed shot die compositions of the present invention. In general, We have found that the use of smaller mesh nepheline syenite results in the mold having a smaller coefiicient of thermal expansion and, conversely, the use of larger mesh nepheline syenite results in a mold having a greater coefficient of thermal expansion.

As previously mentioned, an aqueous solution of sodium silicate or waterglass and calcium aluminate are used conjointly to bind the ceramic mold compositions used in the process of the present invention. In the preferred embodiment of the present invention, we contemplate the use of a viscous, aqueous solution of sodium silicate having an NA O content ranging, by weight, from about 14.8% to 15.2% and SiO content ranging, by weight, from about 28% to 29% with the balance being water. In the preferred embodiment of the present invention, the calcium aluminate cement which is used preferably is in the form of a finely divided powder having a mesh size of less than 100 mesh. Most commercially available grades of calcium aluminate cements having a calcium aluminate content, by weight, in excess of about are suitable for use in the present invention. A typical chemical analysis of calcium aluminate which is suitable for use in forming a mold embodying the present invention is listed in Table II below:

In forming the mold of the present invention, the sodium silicate and the calcium aluminate binders are mixed in a suitable mulling apparatus with the nepheline syenite sand. Preferably, the nepheline syenite sand and calcium aluminate powder are added to the mulling machine and mulled for about 1 /2 to 3 minutes to thoroughly mix the powders. The sodium silicate solution preferably is next slowly added to the powdered mixture and mulled for about 2 to 3 minutes until a moist, sandy mass is formed.

The ceramic mixture is subsequently poured into the cavity 14 of the mold assembly to form the mold 22. The mixture is most advantageously added in small separate portions with each portion being firmly packed in the cavity by using any suitable device, such as a pneumatic rammer, until the last portion is added and packed so that the cavity 14 is completely filled. The progressive packing after each portion of the ceramic mixture is added serves to eliminate voids in the cavity of the mold assembly. In the preferred embodiment of the present invention, when relatively large articles or dies are being cast, the thickness of the ceramic mixture between the walls 10 of the mold assembly and the model 16 preferably ranges from about 6 inches to 12 inches, and the thickness of the mold between the top of the cavity and the model preferably ranges from about 2 /2 inches to 6 inches to minimize the temperature gradient across the mold walls, thereby preventing cracking of the mold during the die forming process. These dimensions are not too critical when relatively small articles or dies are being made by the process of the present invention using a relatively small mold.

Prior to curing the sand mixture, the excess sand at the top of the cavity 14 preferably is leveled off with the top surface of the cavity walls 10 by any suitable means, such as by scraping the excess sand off with a straight edge. A shallow cylindrical opening 24 and a plurality of shallow elongated grooves 26, which extend radially from the opening, may be formed in the leveled top surface 28 of the mold 22 directly above the model 16 in the cavity 14. The opening and grooves facilitate the heating of the mold and the die composition when large dies are being cast by providing a path for the distribution of the furnace atmosphere gas through the die composition and Walls of the mold cavity during the heating cycle of the casting process, which will hereinafter be more fully described. Also, as will hereinafter be more fully explained, smaller dies or castings which are made using smaller molds do not necessarily require gas flow through them, except to reduce the moisture content of the mold.

Since it is desirable to concentrate the path of the furnace gas through the mold cavity, an asbestos rope seal 30 or similar sealing material, which can withstand high temperatures, preferably is positioned in a groove provided in the top surface 28 of the'mold 22 surrounding the opening 24 and the grooves 26. The seal 30 serves to prevent the furnace atmosphere gas from being drawn from the furnace between the top surface 28 of the ceramic mold and the rectangular metal base plate 32 which is used to support the mold in the furnace during the heating cycle of the casting process. The metal base plate 32 may be made of steel or other suitable material which is capable of withstanding the high temperatures employed in the furnace.

The base plate 32 may be affixed to the top of the mold assembly by any suitable means, such as tie studs 34, which extend from the top surface of the walls of the mold assembly into openings 36 provided in the base plate. Preferably, the base plate is positioned so that a flush fit is obtained between the plate and the leveled top surface of the sand mixture. The base plate 32 also is provided with a cylindrical opening 38 which extends through the top and bottom surfaces of the plate so that it is coaxially aligned and in communication with the cylindrical opening 24 in the top surface 28 of the sand mixture. As previously mentioned, hot furnace atmosphere gas is drawn through this opening during the casting process.

Prior to afiixing the metal base plate on top of the mold assembly and ceramic mixture, the surface of the base plate which contacts the mixture preferably is treated with a protective coating to prevent oxidation of the base plate and sticking of the mold to the base plate during the casting process. We have found that a suitable ceramic coating for this purpose is commercially marketed under the trade name of Fiberfrax QF180. This material is a viscous suspension of finely divided, powdered inorganic materials in water and has a total solids content of about 69% by weight. The total solids content of the material consists essentially, by weight, of about 57% silicon dioxide, about 41% alumina and the balance including small quantities of sodium oxide, boron oxide, magnesium oxide and traces of other inorganic materials.

After the base plate has been affixed to the mold assembly, the mold assembly may be inverted by any suitable means so that the ceramic mixture rests firmly on the metal base plate 32. The mixture preferably is allowed to cure at room temperature for an initial curing period of about 1 to 2 hours with the mold assembly in place. The mixture is substantially cured during this initial curing period and forms a relatively firm mold. After the mold is substantially cured during this initial curing cycle, the wooden members of the mold assembly and the pattern or model 16 are removed by any suitable means. Preferably, the mold is covered with a plastic sheet, such as polyethylene, and allowed to stand for about 48 hours to complete the cure. The polyethylene sheet serves to retain moisture in the mold, thereby aiding the formation of chemical bonds which harden the mold during the mold curing cycle.

The chemical reactions which take place during the curing cycle between the calcium aluminate and sodium silicate binders and the nepheline syenite sand base are very complex and not too well understood. However, it is believed that the calcium aluminate causes the sodium silicate to set up and form alkaline earth metal-sodium silicate bonds in the ceramic mixture. The cured mold thus formed has excellent strength and high resistance to cracking when used in the process of the present invention to form the precision cast-to-size brazed shot dies of the present invention.

After the mold cure is completed, the outer side walls and top surfaces of the mold preferably are treated with a suitable heat-resistant ceramic coating to make these surfaces substantially impervious to the passage of the furnace atmosphere gas from the furnace through these surfaces during the heating cycle of the casting process. The upper wall portion of the mold cavity also preferably is treated with a ceramic coating. However, the

lower face portion of the mold cavity preferably is not treated so that the furnace atmosphere gas may pass through this portion of the mold cavity during the heating cycle of the casting process, as will hereinafter be more fully described.

In the preferred embodiment of the present invention, we contemplate the use of a ceramic coating material consisting, by weight, of about 17% olivine flour, 25% kaolin powder and about 58% sodium silicate. The olivine flour preferably has a mesh size ranging from mesh to about 220 mesh. The kaolin powder preferably has a mesh size of less than 350 mesh. The sodium silicate used in the coating preferably has the same composition as the previously mentioned sodium silicate composition used in forming the mold. Of course, it will be appreciated that other ceramic coating materials may be used to coat the mold surfaces for the above-mentioned purposes in accordance with the process of the present invention.

The ceramic coating mixture in the preferred embodiment of the present invention may be prepared by mixing the ingredients in any suitable mixing apparatus until a viscous mixture is formed. The mixture is then applied to the above-mentioned mold surfaces by any suitable means, such as a paint brush, until these surfaces are substantially impregnated with the coating and are relatively impervious. In this manner, the flow of furnace atmosphere gas through the mold during the heating cycle of the die making process is substantially confined to a path through the bottom surface of the mold cavity and the opening 24 in the bottom of the mold. Also, we have found that this coating increases the strength of the mold against cracking.

FIGURE 2 of the drawing illustrates the method by which the precision cast-to-size brazed shot dies of the present invention are made. The cured ceramic mold 22 i positioned on the metal base plate 32 with the mold cavity 39 opening upwards. The base plate is suitably mounted on pedestals 40 within an enclosed furnace 42 which may be heated by any suitable means, not shown in the drawing.

As previously mentioned, the brazed shot die composi tions of the present invention have compensatory thermal shrinkage chanacteristics relative to the thermal expansion characteristics of the mold compositions of the present invention. In casting relatively large dies by the precision casting process of the present invention, we contemplate the use of relatively small metal shot particles and relatively large metal filler particles. When relatively small dies or metal articles having the size of about a six-inch cube or less are made by the process of the present invention, the metal filler particles may be omitted from the die composition.

In the preferred embodiment of the present invention for making relatively large dies or metal articles, we con template the use of steel shot having a mesh size ranging from about 40 to 200 mesh and filler particles consisting of small diameter steel balls ranging in diameter from approximately inch to A2 inch. The shot and balls may be made of any suitable steel, such as SAE 1095 steel, or other suitable metalv It should be appreciated that the size, shape and composition of the shot and filler particles is not limited to the size, shape and composition of the shot and filler particles of the preferred embodiment of the invention. In other words, these characteristics of the shot and filler particles may be varied if corresponding changes are made in the mold composition so that the thermal shrinkage characteristics of the die composition are compensatory relative to the thermal expansion characteristics of the mold in accordance with the process of the present invention. Preferably, the shot is sufliciently small so that capillary action occurs when the molten brazing alloy is poured into the mold cavity to braze the shot and particles together, as will hereinafter be more fully explained. A

typical analysis of steel shot used in the preferred embodiment of the present invent-ion is listed in Table III below:

TABLE III Linear mesh dimenslon in inches Percent shot retained on screen Screen mesh size Prior to placing the shot and balls in the mold cavity, they preferably are thoroughly cleaned by any suitable means to remove dust and other impurities which may interfere with the brazing process. When the process of the present invention is used to form a sheet metal forming die, and since the brazed shot material has superior qualities as a draw die surface than the relatively large filler particles, it is desirable to keep the filler material away from the working surface of the die which is made for such an application. Thus, after the shot has been cleaned, a portion of shot may be added to the mold cavity 39 to cover the lower face 43 of the cavity when the die is to be used in a sheet metal forming press assembly.

In the preferred embodiment of the present invention, the

layer of shot 42 so added preferably ranges in thickness from about 5 inch to about 1% inches. Another advantage of providing this layer of small diameter shot on the lower portion of the die is that it facilitates the machining and finishing of the die after the casting process is completed.

After the layer of shot 42 has been added to the lower face of the cavity, a portion of the steel ball filler particles is poured into the mold cavity over the layer of shot to fill a part of the upper portion 44 of the mold cavity. Subsequently, a portion of the small diameter shot is also added to the upper portion of the cavity to fill the voids between the balls. The steel balls and shot most advantageously are poured into the cavity in successive alternate layers until the cavity is filled. The shot and ball may be vibrated after each addition by any suitable means to insure that they are tightly packed in the mold cavity. In addition to the strength and shrinkage characteristics they impart to the dies, the balls also are a relatively cheap filler material and substantially reduce the cost of producing the larger sized brazed shot dies of the present invention.

Thermocouples 46 preferably are inserted in the mold cavity and the mold body in any suitable manner to measure the temperature of the mold and the die compositions during the die forming process. The thermocouples may be of any suitable type which will give relatively accurate temperature readings in the relatively high temperature ranges employed in the die forming process. The thermocouples may be connected by lead wires 48 to an instrument box 50 located outside of the furnace so that temperature readings of the die composition and the mold may be recorded during the die forming process.

The brazed shot die which is formed by the process illustrated in FIGURE 2 is typical of a punch portion of a sheet metal forming die. When a punch die is used in a sheet metal press assembly, it is usually affixed to a movable member in the sheet metal press assembly by welding or mechanically locking the die to the member. In the present invention, an extension member for positioning the die in a sheet metal press assembly may be partially embedded in the shot and balls in the mold cavity so that when the die is cast the extension member and the die are formed into an integral punch unit.

For instance, FIGURE 2 of the drawing shows a hollow generally cylindrical metal extension member 52 partially embedded at its lower end in the shot and balls located in the mold cavity. The extension member has a flat, annular radial flange 54 affixed to its upper edge and coaxially aligned with cylindrical walls of the member, which may be used to secure the punch unit to the sheet metal press assembly. The lower portion 56 of the extension member, which is embedded in shot and balls, preferably is bent inwardly so that the extension member will not pull out of the die when the die is removed from the mold after the casting process is completed. Of course, the extension member should be made of a material, such as steel, which is capable of withstanding the temperatures used in the die making process. Thus, the step of atfixing the die to a structural member to form a punch portion of a sheet metal press assembly may be eliminated by making the die and extension member into an integral unit by the process of the present invent-ion. Also, the extension member may be used to pull the die from the mold when the casting process is completed.

As previously mentioned, the enclosed furnace 42 illustrated in FIGURE 2 of the drawing may be heated by any suitable means, not shown in the drawing, to raise the temperature of the furnace atmosphere gas which is circulated through the furnace during the die making process of the present invention. As shown in FIGURE 2, a gas generator 58 for producing the furnace atmosphere gas and a pump 60 for pumping the furnace gas from the generator through a conduit 62 into the furnace are suitably located outside of the furnace. The furnace also is provided with a conduit 64 for venting the furnace atmosphere gas from the furnace during the die making process. The furnace gas from the generator preferably is continually circulated in the furnace during the die forming process at a pressure slightly above atmospheric pressure to prevent oxygen from the atmosphere from entering the furnace.

Preferably, a reducing atmosphere is used in the process of the present invention to reduce the metal oxides on the shot and steel particles of the die composition and to prevent oxidation of the brazing alloys during the brazing cycle, as will hereinafter be more fully explained. In the preferred embodiment of the present invention, we contemplate the use of a modified dissociated ammonia gas as the furnace atmosphere in the die making process of the present invention. The hydrogen concentration of the dissociated ammonia furnace gas preferably is varied during the die forming process from a maximum, by volume, of 15% to a minimum of 2%, as will hereinafter be more fully explained. Therefore, the gas generator 58, which is used in accordance with the process of the present invent-ion, preferably is capable of varying the hydrogen concentration of the furnace gas which is supplied by the generator to the furnace.

We have also found it desirable in most cases to add a small amount of natural gas to the furnace atmosphere during a certain stage of the seating cycle of the die making process, as will hereinafter be more fully explained. The addition of the natural gas assists in removing oxygen and Water vapor released from the mold and furnace walls. It also reduces carbon diffusion from the shot partioles when steel shot is used and/ or replenishes carbon in the steel shot particles if the carbon is depleted. The natural gas may be introduced into the furnace by any suitable means, such as introducing it through the pump into the generator gas stream which is fed into the furnace. Preferably, the natural gas concentration should range from about 4% to 5% of the total furnace atmosphere, although the concentration may be varied somewhat according to the conditions in the furnace.

As previously mentioned, the furnace atmosphere gas is circulated through the mold and die composition during the heating cycle of the die making process of the present invention. In this manner the mold, shot and filler balls are heated, and the mold cavity is expanded to a sufficient degree prior to introducing the brazing alloy into the mold cavity during the die making process. As shown in FIG- URE 2 of the drawing, a conduit 68 is suitably afiixed to the opening 38 in the base plate 32 to vent the gas which passes through the mold 22 from the furnace. The gas may be drawn through the mold and conduit 68 by any suitable means, such as suction created by a compressor 70 located outside the furnace. The hot furnace gas is preferably cooled by means of a suitable heat exchange 72 before entering the compressor. Another conduit 74 may be used to recycle the gas from the compressor into the furnace 42. The hot recycle furnace gas from the compressor may be cooled by means of a suitable condenser 76 before being recycled into the furnace.

It is desirable to minimize the moisture content of the gas circulating in the furnace to prevent oxidation of the steel shot and balls during the heating cycle of the die making process. When the circulating furnace gas passes through the mold, it picks up moisture from the mold. Therefore, a suitable water trap 78, which functions conjointly with the condenser 76, may be provided in the conduit line 74 to remove any entrained moisture in the furnace gas prior to recycling the gas from the compressor to the furnace through the conduit 74.

The brazing alloys, which are used to form the brazed shot dies of the present invention, may be introduced into the mold cavity during the die making process by any suitable means. For instance, the brazing alloy may be melted by any suitable means and poured into a metal receiving funnel 80 which is suitably located outside the furnace 42. The funnel 80 is most advantageously located above the level of the mold 22 so that during the die making process, as will hereinafter be more fully explained, the molten alloy will flow by force of gravity through a conduit 86, which is suitably connected to the funnel 80, into another metal receiving funnel 38, suitably positioned above the mold cavity in the furnace. The receiving funnel 88, through which the molten brazing alloy flows into the mold cavity, may be suspended above the mold cavity by any suitable means, not shown in the drawing.

It will be appreciated, of course, that brazed shot dies may be produced in accordance with the process of the present invention by using any suitable brazing alloy which will result in the formation of a precision cast-tosize shot die. An example of such an alloy is a commercially available product composed of approximately 60%, by weight, copper, 39.25% by Weight of zinc and 0.75% by weight of tin. However, in the preferred embodiment of the present invention, we contemplate the use of brazing alloys consisting essentially, by weight, of about 35% to 55% copper, about 25% to 40% zinc, about 5% to 25% manganese, about to 10% nickel, about 0% to aluminum, about 0% to 3% silicon, about 0% to tin, and about 0% to 5% antimony. The alloys of the above composition ranges also may have trace quantities of other metals, such as prosphrous, cadmium and lead. Brazing alloys made from these compositions have a melting temperature ranging from about 1420 F. to about 1650 F. A typical analysis of a brazing alloy which is suitable for use in making a brazed shot die in accordance with the process of the present invention is listed in Table IV below:

TABLE IV Alloy component: Percentage, by wt. Copper 45 Zinc 29 Manganese l5 Nickel 5.4

Tin 5.3

Silicon .2 Phosphorous maximum .01 Cadmium do .04 Lead do .05

After the mold cavity 39 has been filled with the steel shot and balls in the above-described manner, the mold 22 is placed in the furnace, and the punch extension member 52 and thermocouples 46 are suitably installed. The furnace preferably is gradually heated at a rate of about 30 F. per hour in a modified dissociated ammonia furnace atmosphere containing about 2.5% by volume, of hydrogen, with the balance being nitrogen, at a pressure slightly above atmospheric pressure. During the initial heating stages of the die making process, it is desirable to maintain the hydrogen concentration of the gas from the generator at this level to prevent an explosive gas mixture from forming in the furnace.

When the furnace starts to heat, the compressor is cut in with sufficient intake vacuum to draw the furnace gas through the mold cavity 39 and mold 22, thereby heating the shot, balls and mold to cause the mold cavity to expand. Of course, as previously mentioned, furnace gas is continually supplied to the furnace 42 by the generator 58 and vented from the furnace through the vent conduit 64 during the entire die making process. As the furnace is gradually heated, the temperature differential between the mold and the furnace atmosphere preferably is maintained within a maximum temperature differential of about F. until the furnace atmosphere reaches about 350 F. to prevent the mold of the preferred embodiment of the present invention from cracking. As the furnace temperature is raised above 350 F., the temperature differential may be maintained within a maximum limit of about 300 F. As the furnace temperature is raised to about 1200 F., natural gas is preferably introduced into the furnace so that the concentration of the natural gas is about 4% of the total furnace atmosphere. When the furnace temperature reaches about 1400 F., the hydrogen content of the furnace atmosphere gas preferably is increased to a concentration ranging from abuot 10% to about 15%, by volume, to reduce any metal oxides on the steel shot and balls in the mold cavity. At about 1550 F., the natural gas feed to the furnace atmosphere preferably is turned off.

The heating of the furnace is continued, and the relatively high hydrogen concentration gas is recycled through the mold by the compressor until the metal oxides on the shot and balls have been reduced and the mold, shot and balls have reached a minimum temperature of about 1650 F. and a maximum temperature of about 1725 F., as indicated by the thermoscouples 46 on the instrument box 50. In this temperature range, the mold composition of the preferred embodiment of the invention expands so that the mold cavity becomes enlarged. Since the thermal expansion characteristics of the mold are so similar to the thermal shrinkage characteristics of the metallic die composition in the preferred embodiment of the invention at the brazing temperature, the degree of expansion of the cavity is not too critical. However, the expansion of the mold cavity during heating must compensate for the size change of the metallic die composition during cooling in accordance with the process of the present invention. Thus, when the furnace is cooled during the cooling cycle of the die making process, the brazed shot die composition of the preferred embodiment of the invention contracts in the mold cavity to the desired final shape and size of the die to be produced, as will hereinafter be more fully explained.

After the mold, shot and balls have been heated to a temperature within the above-mentioned temperature range and the dew point of the furnace has dropped to a maximum of about 10 F., indicating that the steel shot and balls have been adequately cleaned, the compressor is cut off. The furnace preferably is heated at this temperature for approximately four hours, and the hydrogen concentration of the furnace gas circulated through the furnace by the gas generator 58 preferably is changed to about 2.5% by volume, in preparation for the brazing step, as will hereinafter be more fully explained. We have found that if hydrogen concentration of the furnace gas is maintained at about 2% to 3%, by volume, a more satisfactory braze is obtained in the die making process of the present invention using the brazing alloys and the shot and balls of the preferred embodiment of the present invention.

After the furnace has been maintained at a temperature of about 1700 F. for about four hours in a furnace atmosphere having a hydrogen concentration of about 2.5%, the molten brazing alloy is poured into the funnel 80 so that it flows through the conduit 86 to the receiving funnel 88 and into the mold cavity 39. The brazing alloy of the preferred embodiment of the present invention may be melted in any suitable receptacle by any suitable means, not shown in the drawings. Preferably, the alloys of the present invention are melted in the presence of anhydrous borax which acts as a flux and prevents the oxidation of the alloys during the melting step. Prior to adding the brazing alloy to the mold cavity, the funnel 80, conduit 86 and receiving funnel 88 may be treated with a protective coating, such as Fiberfrax QF-180, to prevent the accumulation of the brazing alloy on these members.

As the molten alloy gradually flows into the mold cavity during the brazing cycle, a capillary action occurs, and the alloy is dispersed throughout the cavity to completely fill the voids in the cavity between the shot and balls. In adding the brazing alloys of the preferred embodiment of the present invention to the mold cavity, the. temperature of the brazing alloy should not exceed 1650 F. prior to entering the mold cavity to prevent the zinc in the alloys from vaporizing. As the brazing alloy is added to the expanded mold cavity, an alloy action occurs in the presence of the steel shot and steel balls in the cavity, and the alloy tends to solidify to form the brazed shot die composition of the present invention.

After suflicient brazing alloy has been added to the mold cavity, the furnace is allowed to cool at a predetermined rate, preferably about 30 F. per hour, in a reducing furnace atmosphere containing about 2.5 by volume, of hydrogen until the mold temperature is cooled to about 1000 F. to complete the brazing step. When the mold has reached this temperature, the furnace heat and gas from the generator most advantageously are turned off, and the die composition is allowed to cool normally to room temperature.

As previously mentioned, on cooling to room temperature, the die composition of the present invention contracts in the mold cavity to substantially the same initial size and shape of the mold cavity. Thus, precision cast-to-size brazed shot dies can be made by the process of the present invention which substantially reduces the number of post-machining operations required to conform the die to the desired final shape.

The following is a specific example of a precision castto-size brazed shot die produced by the above-described process of the preferred embodiment of the present invention using the preferred mold and die compositions:

Example A wooden mold assembly was prepared for forming a ceramic mold having a width of 28 inches, a length of 52 inches and a height of 19.5 inches. A rectangular solid plaster model having a width of 15 inches, a length of 39 inches and a height of 16 inches was centrally positioned in the bottom of the mold assembly so that the side walls of the mold cavity of the mold, which was produced in accordance with the above-described process of the present invention, had a thickness of 6.5 inches and a bottom wall thickness of 3.5 inches. The mold was formed using 1200 pounds of nepheline syenite sand, 72 pounds of sodium silicate and 84 pounds of calcium aluminate. The chemical composition of the nepheline syenite and calcium aluminate which was used to form the mold is listed above in Table I and Table II, respectively. The sodium silicate which was used had a chemical analysis, by weight, of about 15% SiO about 28% Na O and the balance water. After the mold was cured and the model was removed to form the mold cavity, the walls of the mold and the side walls of the mold cavity were treated in the above-described manner with a ceramic mixture consisting, by weight, of about 17% olivine flour, about 25% kaoline flour and about 58% sodium silicate.

A one inch layer of SAE 1095 steel :shot having a mesh size ranging from 40 to 200 mesh was added to the bottom face of the mold cavity. Alternate layers of /s-inch diameter SAE 1020 steel balls were added to the mold cavity in the manner previously described to fill the cavity. A total of 321 pounds of steel shot and 395 pounds of steel balls were added to the cavity. The mold was then placed in a suitable furnace and heated in an atmosphere of hydrogen, nitrogen and natural gas to a temperature of 1700 F. under the process conditions previously described in reference to the preferred embodiment of the present invention. A total of 281 pounds of a brazing alloy having a composition as listed in Table IV above was poured into the cavity, and the die composition was then cooled to room temperature under the process conditions previously described. The precision cast-to-size brazed shot die produced in this manner had substantially the same size and shape as the initial size and shape of the model used to form the mold cavity.

Referring to FIGURE 3 of the drawing, a completed integral punch portion or unit of a sheet metal forming die is illustrated which may be produced by the process of the present invention, as shown in FIGURE 2 of the drawing. The punch unit 90 consists of a precision cast-tosize brazed shot die member 92 and the generally cylindrical extension member 52. The punch unit may be used in a suitable sheet metal forming press assembly.

The precision cast-to-size brazed shot dies produced by the process of the present invention offer several advantages over iron or steel dies which are cast by conventional iron and steel casting processes. Also, the die compositions and mold compositions of the present invention, when used in accordance with the process of the present invention, make possible the manufacture of precision cast-to-size brazed shot dies. Therefore, the number of post-machining operations required to conform the die to the desired final shape and size is substantially reduced, thereby greatly decreasing the costs involved in forming metal dies. In addition, the brazed shot dies produced by the process of the invention have excellent wear resistance when compared with most cast iron or steel dies produced by a conventional casting technique.

While we have described our invention in terms of certain preferred embodiments, it is not to be limited thereby, and it should be understood that other variations may be apparent to those skilled in the art and are within the intended scope of the invention as defined by the following claims.

We claim:

1. A metallic brazed shot die for use in a sheet metal forming press assembly, said die comprising a metal body and a thin, contoured, wear-resistant work face layer, said body being formed of relatively small diameter steel shot particles intermingled with metal filler particles and bonded together by a brazing alloy, said work face layer being formed of said shot particles bonded together and to said body by said alloy.

2. A metallic brazed shot die for use in a sheet metal forming press assembly, said die comprising a metal body and a contoured wear-resistant work face layer, said body being formed of relatively small diameter steel shot particles intermingled with steel filler particles of larger size and bonded together by a brazing alloy, said work face layer being formed of said shot particles bonded together and to said body by said alloy, said layer having a maximum thickness of about 1.5.

3. A metallic brazed shot die for use in a metal forming press assembly, said die comprising a metal body and a contoured wear-resistant work face layer, said body being formed of relatively small diameter steel shot particles intermingled with steel filler particles of larger size and bonded together by a brazing alloy, said alloy comprising,

1 5 by weight, about 35% to 55% copper, about 25% to 40% zinc and about 5% to 25 manganese, said work face layer being formed of said shot particles bonded together and to said body by said alloy, said layer having a thickness ranging from /2 to 1 /2".

4. A metallic brazed shot die for use in a metal forming press assembly, said die comprising a metal body and a contoured Wear-resistant Work face layer, said body being formed of steel shot particles intermingled with spherical steel filler particles and bonded together by a brazing alloy, said shot particles having a mesh size ranging from about 40 mesh to +200 \mesh, said filler particles having a diameter ranging from about A" to /2", said alloy consisting essentially, by Weight, of about 35% to 55% copper, about 25 to 40% zinc, about 5% to 25% manganese, about 0% to 10% nickel, about 0% to 10% tin, about 0% to 5% aluminum, about 0% to 5% antimony, about 0% to 3% silicon, said work face being formed of 16 said shot particles bonded together and to said body by said alloy, said layer having a thickness ranging from about /2" to 1 /2.

References Cited UNITED STATES PATENTS CHARLES W. LANHAM, Primary Examiner.

G. P. CROSBY, Assistant Examiner.

US. Cl. X.R. 76-107 

